Reciprocating Compressors

A reciprocating compressor is a mechanical device for increasing the pressure of a natural gas stream. Increased pressure is needed for the treatment or processing of natural gas and for promoting the movement of natural gas from production, gathering and boosting, and processing sites to customer distribution systems. Reciprocating compressors vent natural gas from piston rod packing systems during normal operation. The rod packing systems are designed to have a tight enough fit around the piston rod to reduce leakage; however, the systems cannot entirely eliminate leakage without binding the rod. During extended operation, the rod packing wears, and emissions increase over time.  

Note that other components connected to the compressor, such as flanges, valves, and fittings, tend to leak more due to thermal and mechanical stresses associated with the compressor operation. For information about these emissions see the Equipment Leaks page. This page focuses on emissions from rod packing systems.

A reciprocating compressor, or positive displacement compressor, uses a plunger or piston to compress gas in the compressor cylinder as the engine drives the piston back-and-forth inside the cylinder. The engine turns the crankshaft, which connects to the piston rod via a “crosshead” that converts the engine rotating motion to the piston reciprocating motion. Piston rod packing systems are mounted where the rod passes through the inboard cylinder head and are used to maintain a relatively tight seal around the piston rod, preventing the compressed gas in the compressor cylinder from leaking, while allowing the rod to move freely. Reciprocating compressors often use multiple cylinders with gas moving from the inlet low pressure to intermediate pressure, and then to high pressure. Figure 1 shows a typical rod packing for reciprocating compressors.

Piston rod packing consists of a series of flexible rings that fit around the piston rod to create a seal against gas leakage. The packing rings are lubricated with circulating oil to reduce wear, help seal the unit, and draw off heat. Packing rings are held in place by a set of packing cups, normally one for each pair of rings, and kept tight against the rod by gas pressure. The number of cups and rings will vary depending on the compression chamber pressures. A “nose gasket” on the end of the packing case prevents leaks around the packing cups. 

Reciprocating compressors vent natural gas from piston rod packing systems during normal operation. The rod packing systems are designed to have a tight enough fit around the piston rod to reduce leakage, but not so tight as to bind the rod and cause faster wear. Thus, the packing cannot entirely eliminate leakage from the inboard side of the cylinder. Under the best conditions, new packing systems properly installed on a smooth, well-aligned piston rod can be expected to vent a minimal amount of gas. Higher vent rates from rod packing are a consequence of fit, alignment of the packing parts, and wear. Rod packing emissions typically occur from four areas: 

• Around the rings from slight movement in the cup groove as the rod moves back and forth
• Between the rings and rod, increasing with wear on the rings and rod
• Around the packing case through a worn or damaged nose gasket
• Between damaged or fouled packing cups, which are typically mounted metal-to-metal against each other

Gases are vented to the atmosphere either through an open “distance piece” connecting the cylinder to the “cross head” and “crank shaft,” or through a vent stack connected to the packing flange.

Figure 1. Rod Packing for Reciprocating Compressor.

U.S. Environmental Protection Agency. (2014, April). Oil and natural gas sector compressors – Report for Oil and Natural Gas Sector Compressors Review Panel. https://www.ourenergypolicy.org/wp-content/uploads/2014/04/epa-compressors.pdf